Sheet metal tubing is typically used in ventilation systems designed to move quantities of air or other gases from one point to another. A known machine for producing helically wound lock-seam tubing is disclosed in British patent publication GB 2,213,748 (see FIG. 1) which is incorporated herein by reference. In this machine, a metal strip is fed to a forming head by means of drive rollers, and a helically wound lock-seam tube is formed by clinching rollers associated with the forming head.
Another known machine for producing helically wound tubing is disclosed in British patent specification GB 1,168,178, wherein FIGS. 3 and 4 show different seams.
The technology related to such machines is well-known to those skilled in the art. At present, several further developed machines of a similar type are available on the market. However, these machines still suffer from some specific drawbacks.
In the production of helically wound lock-seam tubes of the present type, and of straight-seamed lock-seam tubing, the lock-seam tends to slip, which results in an undesirable change in tube diameter. Moreover, when tubing which is circular in cross-section is ovalized, a similarly undesirable side-slip is usually encountered. The conventional solution to this problem is to increase the pressure of the clinching rollers acting on the lock-seam in the forming head. However, such an increased clinching pressure can distort the seam and, thus cause damage to, and deficiencies in, the lock-seam as well as in the adjacent portions of the metal strip from which the tube is formed. Thus, the adjustment of the clinching pressure is crucial to the quality of the lock-seam. This clinching pressure is difficult to control, and is thus not an effective method for controlling side-slip in lock-seamed tubing.
In addition to the clinching pressure, the operator running the machine must consider several other parameters, such as the thickness of the metal strip, the lubrication of the same, and possible wear of different machine components, for example the rollers. Thus, great demands are made on the skill and experience of the operator.
In spite of the operator's skill, it is hard to avoid slip or sliding in the lock-seam. An undesirable increase of the tube diameter can occur either in production, immediately after the forming head, or during storing, handling and transportation of the tubes. It should be noted that the tube diameter may also be undesirably reduced due to lock-seam slip, for example when tubes are subject to pressure forces from the outside.
The change in tube diameter is always a problem, since the tubes delivered do not fulfill the customers' specifications and requirements. Specific problems are encountered when using the tubes in ventilation duct systems mounted on different premises. For example, vibrations caused by fans and similar equipment can initiate lock-seam slip which, in turn, leads to undesirable leakage of air and pressure drops. In severe cases, lock-seam slip of the ventilation ducts can jeopardize safe function of the whole ventilation system.
In ventilation duct systems, different fittings are connected to the helically wound lock-seam tubes forming the major part of the system. Such fittings are bends, T-pieces, dampers, sound attenuators, etc. The fittings are normally inserted in the end of the tubes, and a sealing ring on the inserted portion of the fitting ensures safe sealing. However, if the tube has an undesirably increased diameter, there is a risk that sufficient sealing cannot be obtained in the joint between the fitting and the tube. This may lead to the leakage and pressure drop problems discussed above, resulting in energy losses and increased running costs.
In other cases, the fitting is not provided with a pre-mounted seal, but the joint between the outside of the fitting and the inside of the tube is filled with a mastic for sealing purposes. If the gap is too large due to an undesirable increased tube diameter, sufficient sealing may not be obtained by means of the mastic. Even if the mastic is safely fastened, problems may arise later due to increased tube diameter caused by vibrations, air pressure peaks, etc.
Further, change in tube diameter is disadvantageous when prefabricated annular fastening devices and the like are to be applied on the outside of the tube: Such devices do not fit if the tube diameter has changed.
An attempt to deal with these problems is found in published U.S. Patent application 2001/0013375 to Lennartsson. Lennartsson '375 discloses a method for producing helically wound lock-seam tubing which is further stabilized by crimping the lock-seam and expanding the lock-seam transversely by means of a transverse roller system. The method of Lennartsson, however, introduces another type of instability into the lock-seam by expanding the entire transverse portion of the seam, which distorts the seam to a disadvantageous extent. This distortion leads to instability and the necessity for using string or sealing means within the lock-seam itself. There is thus a need for lock-seam tubing which is not deformed completely across its entire seam while possessing an increased stability with respect to side-slip. There are similar needs for apparatus and methods for producing such stabilized lock-seam tubing.